In some communication systems, a common bus is used for establishing communication connections. Such a system is shown in U.S. Pat. No. 4,150,259 dated Apr. 17, 1979 which patent is incorporated herein by reference as if it had been reproduced in full. In such systems when a call is to be established the calling station becomes connected to an assigned link. If the call is one which is directed to a separate switching network a connection is made to that network via a central office or PBX line. That line is then also connected to the same assigned link. The calling station then dials, using the dual tone multifrequency (hereinafter referred to as MF tones) key pad at the local station, and the MF tone signals are communicated to the central office via the common connection on the assigned link.
The MF dialing at the station set consists of a series of bursts of MF signals of widely variant duration and spacing which correspond to button depressions on the telephone dial. These are delimited by periods of silence or the absence of MF signals. Each valid MF signal burst is registered by a MF receiver as a dialed digit. In the modern communication system, these digits are collected for interpretation by a common control circuit. The transmission path from station set to MF receiver is established via a voice switching network in the communication system.
Problems result, however, when the central office switching machine is one which accepts only rotary dialed (pulsed) digits. These problems are compounded when the central switching machine can accept a mixture of rotary dialed and tone signalled digits.
Primarily, the problems come about because the local communication system, which receives MF tones from the stations, converts the tones into conventional pulsed digits. In such situations the tip and ring leads of the Central Office line are pulsed by a relay contact placed in the communication path. The relay operates under control of the local communication system processor. Thus, if the central office switching machine is one where both tone and rotary pulsed signals are accepted then the local system pulsing relay serves to periodically interrupt the tone of each digit, (as received from the station), thereby sending false digits to the central office.
For example, assume the tones for the digit four are placed on the line by the station. Also assume that the local communication system is arranged to pulse the central office line by opening and closing (pulsing) the transmission line four times in response to the received digit four. Then, in such a condition, the central office (which accepts tone signals) will register four occurrences of the digit four. If a five were being transmitted then the central office would see five fives. The reason for the multiplicity, of course, is the fact that the digit pulsing relay interrupts the tone signal and the central office electronic detectors respond fast enough to record multiple digits.
In addition, the fact that a relay in the line circuit is pulsing causes transient problems in the local MF tone receiver, as discussed below.
One solution to the problem is to isolate the line from the station during the transmission interval so that the false tone signals would not be sent forward to the central office and so that the transient signals would not be sent back to the local stations. However, this is not possible since there are situations during the dialing interval where it is mandatory for the calling subscriber to hear call progress tones from the central office. One example of this necessity is where the subscriber dials an access code and must wait to hear a second dial tone from the central office before dialing the remainder of the digits.
Attempts to, in some way, interrupt the transmission path between station set and CO solely during periods of MF dial button depression are complicated by the asynchronous nature of dialing. The first evidence of button depression, being the output of a digit to the common control circuit from the MF receiver, occurs after the unwanted MF signal has also been transmitted to the CO. In some prior art arrangements the transmission path to the CO interface is routed in tandem through the MF receiver which, independent of the common control circuit, provides a rapid dissolution of the network path to the CO interface when MF signals are present, or, when, as will be described below, outpulsing is occurring. This method has the disadvantage of requiring two separate network paths during the dialing period with the penalty of reduced network traffic capacity.
A second problem, characteristic of rotary dialing, arises in this conference configuration. In the loop start or ground start signaling traditionally used on CO lines and trunks, dial pulsing is performed by interrupting the flow of DC current normally present in the CO loop during the active or "off-hook" state. A loop closure relay in the CO interface, controlled by the common control circuit, establishes DC continuity to the CO loop. Dial pulses are produced by opening this relay for specified "break" intervals. The number of consecutive break intervals is equal to the value of the digit being dialed. The break intervals are delimited by "make" intervals which are periods of relay closure. Short make intervals signify the concatenation of break pulses within a digit while longer make intervals signify the conclusion of a digit.
The transitions from make to break and vice versa create large amplitude clicks at the CO loop input to the CO interface circuit. The voice-band frequency components of these clicks are naturally transmitted to the MF receiver and station set through the voice switching network. At the receiver, they may cause mutilation of the MF signals which are simultaneously being received. At the station set, these clicks are annoyingly loud and may disturb the station set user. It is necessary, therefore, to inhibit the transmission path inward from the CO interface to the voice switching network during the outpulsing operation and to restore this path as soon as possible upon conclusion of outpulsing so that the station user may receive call progress tones from the CO.